The Court House News Service records this case where a Basquiat the owner valued at $500k was mysteriously sold:

Lio Malca sued Katzuhito Yoshii and his Yoshii Gallery in New York County Court. Malca claims the gallery at 980 Madison Ave. sold his Jean-Michel Basquiat artwork known as “Untitled (Football Helmet)” which Yoshii had in bailment. Malca says he did not consign the work to the gallery, and it was “not offered as collateral against any debts,” but was merely on loan to defendant Yoshii as a favor.  Malca claims that while Yoshii had the piece, he offered to sell it to the gallery but Yoshii refused to pay the asking price of $300,000.  A few days later, Malca says, he asked for the art back, but the gallery told him it had been sold “without title.”

Update: ArtInfo has a very different version of events:

In a phone conversation, Malca told ARTINFO that he had canceled the lawsuit the next day after Yoshii delivered the sculpture […] “We tried to communicate, we didn’t, and then when he got the documents he came back and said here’s your piece,” the dealer said. “I’m actually very close with him, I just couldn’t get in touch with him. And that’s it, that’s it — it’s all been settled and finished.” Maica declined to go into further detail or describe the background of the artwork.